Ysgol Dyffryn Teifi is a bilingual (Welsh and English) comprehensive school in Llandysul, Ceredigion. It stands on the site of the previous Llandysul Grammar School. The school was established in 1984 following the re-organisation of education in the Teifi Valley. The school's history dates back to its period as a grammar school in the 19th century.
In 2001 there were 570 pupils at the school, the numbers then dropped, but had increased once more to 527 in 2007, and were expected to continue to gradually increase. 78% of the pupils were from homes where Welsh was the main language spoken in 2001, this increased to 83% in 2007, with 91% of the pupils able to speak the language to first language standard.
Amongst the ex-pupils are T. Llew Jones, Emyr Llewelyn, Kay Swinburne, Joshua Gerwyn Elias, Aled Gwyn Jones, John Ceredig Evans, Kristian James Davies, John Bowen Jones, Dr Llyr Jones and Ian N Morgan.